The present invention relates to a method of manufacturing optical devices such as an optical low-pass filter (hereinafter called an “optical filter”) and, in particular, to a method of manufacturing an optical filter in which high quality is maintained.
An optical filter is known in the art as a device that is disposed in front of an imaging device such as a camera, to suppress spurious signals and eliminate problems such as chromatic aberration. As the numbers of optical products have increased recently, and also the demands thereon have become more varied, the requirement is for reliable maintenance of the optical characteristics thereof, such as a high transmitted wave front precision.
A method of manufacturing an optical filter in accordance with the prior art is shown in FIGS. 6 to 7C, where FIG. 6 is a vertical section through an optical filter, FIG. 7A is a vertical section through an optical wafer, FIG. 7B is a vertical section through a stack of multiple optical wafers, and FIG. 7C is a plan view after that optical wafer has been cut into individual optical chips.
Various materials such as crystal (including quartz crystal), plastic, and glass (including IR-cutting glass) are used as optical filters. An optical filter is formed as a single chip of an optical material, or as a stack of a plurality of optical chips, such as three optical chips 1a, 1b, and 1c, as shown in FIG. 6. With an optical filter in particular, an optical thin film 2 for cutting infrared rays is formed on both main surfaces of each of the optical chips 1a, 1b, and 1c, and they are bonded together with an adhesive 3.
The usual method is first to cut three optical wafers 1A, 1B, and 1C of a flat-plate shape from an optical material and polish both main surfaces of each of the optical wafers to a mirror finish (see FIG. 7A), then form the optical thin film 2 for cutting infrared rays on both main surfaces of each of the optical wafers. The optical thin film 2 is formed in multiple layers (not shown in the figures) by a method such as vapor deposition, with a first layer of Al2O3, a second layer of ZrO2, and a third layer of MgF2, by way of example.
The three optical wafers 1A, 1B, and 1C having the optical thin film 2 formed thereon are bonded together by the adhesive 3, to form a stacked optical wafer (hereinafter called a “multi-layer optical wafer”) 4. The multi-layer optical wafer 4 is then cut vertically and horizontally to divide it. In general, the optical wafer 4 is cut by a dicing saw that has a rotating blade of fine particles (powder) of diamond (a diamond wheel) affixed to the outer periphery of a thin circular plate. This makes it possible to obtain a large number of optical devices having a stack of the three optical chips 1a, 1b, and 1c (see Japanese Patent Laid-Open Publication No. 6-313811).
However, with the above prior-art method of manufacturing an optical filter, a portion equivalent to the width of the blade edge of the dicing saw is shaved off the optical wafer 4 during the cutting of the multi-layer optical wafer 4 by the dicing saw. Since this means that wasteful shavings portions are generated from the multi-layer optical wafer 4, the utilization efficiency of the optical material is bad. If the optical material is crystal, in particular, the material costs also pile up, which is wasteful. In such a case, the use of a scribing/cutting method (dividing method) that is used in applications such as cutting liquid-crystal panels is drawing attention from the viewpoint of improving the utilization efficiency of the multi-layer optical wafer 4 (the optical material).
In this scribing/cutting method illustrated in FIGS. 8A and 8B, by way of example, a rotary cutter (not shown in the figures) having diamond as the blade edge thereof is pressed against the optical thin film 2 of the optical wafer 1A from above. This provides hairline cracks (scribed grooves or scratched grooves) 5 as division lines on the surface of the optical wafer 1A. Pressure is the applied from above each hairline crack 5 that acts as a division line, by a dividing device (not shown in the figure) such as a breaker fabricated by Mitsuboshi Diamond, to divide the optical wafer 1A. Since this ensures there are no wasteful cut-off portions from the optical wafer 1A caused by the width of the blade edge, as with a dicing saw, the utilization efficiency is increased by approximately 20%. Note that a single the optical wafer 1A is shown in FIGS. 7 and 8.
If this scribing/cutting method is applied to the multi-layer optical wafer 4 (see FIG. 7B) formed of the three optical wafers 1A, 1B, and 1C, however, a problem arises in that it is difficult to split the optical wafer after the hairline cracks (scribed grooves) 5 have been provided, due to the adhesion of the adhesive 3 interposed between the multiple surfaces of the multi-layer optical wafer 4. In addition, the optical thin film 2 can break up finely during the formation of the horizontal and vertical hairline cracks (division lines) 5 from above the optical thin film 2 formed by a method such as vapor deposition on the surface of the multi-layer optical wafer 4. This leads to a problem in that the optical characteristics are adversely affected by the deposition of fine dust on the main surfaces of the optical device after the splitting of the optical wafer. Note that similar fine dust is also generated when a dicing saw is used, but the fine dust due to scribing can remain attached even after washing.
With the dicing saw of the prior-art example, an amount of optical material of substantially the same quantity as the width of the blade edge is shaved off from the optical wafer, as described previously, while frictional resistance is generated between the optical wafer and the rotating blade during the cutting of the multi-layer optical wafer 4. Thus there are stress concentrations in the vicinity of the processed layers that at the positions of the cuts, due to frictional resistance, causing distortion (processing distortion). This processing distortion remains unchanged on the outer peripheral edges of each optical filter after the optical wafer has been divided, reducing the transmitted wave front precision in those areas and ultimately causing deterioration of the optical characteristics.
It is therefore necessary to reduce the effective area and increase the outer planar dimensions of each optical filter. This impedes any effort to make the optical device smaller and also causes an inevitable problem concerning productivity, especially when making the multi-layer optical wafer 4 even smaller. This not only affects these multi-layer optical wafers 4; similar problems occur when forming an optical filter from a single sheet (a single chip) or forming wavelength plates other than optical filters.